[PATCH] backlight: pwm_bl: configure pwm only once per backlight toggle
Michal Vokáč
michal.vokac at ysoft.com
Thu Oct 17 10:59:20 UTC 2019
On 17. 10. 19 11:48, Michal Vokáč wrote:
> On 17. 10. 19 10:10, Uwe Kleine-König wrote:
>> A previous change in the pwm core (namely 01ccf903edd6 ("pwm: Let
>> pwm_get_state() return the last implemented state")) changed the
>> semantic of pwm_get_state() and disclosed an (as it seems) common
>> problem in lowlevel PWM drivers. By not relying on the period and duty
>> cycle being retrievable from a disabled PWM this type of problem is
>> worked around.
>>
>> Apart from this issue only calling the pwm_get_state/pwm_apply_state
>> combo once is also more effective.
>>
>> Signed-off-by: Uwe Kleine-König <u.kleine-koenig at pengutronix.de>
>> ---
>> Hello,
>>
>> There are now two reports about 01ccf903edd6 breaking a backlight. As
>> far as I understand the problem this is a combination of the backend pwm
>> driver yielding surprising results and the pwm-bl driver doing things
>> more complicated than necessary.
>>
>> So I guess this patch works around these problems. Still it would be
>> interesting to find out the details in the imx driver that triggers the
>> problem. So Adam, can you please instrument the pwm-imx27 driver to
>> print *state at the beginning of pwm_imx27_apply() and the end of
>> pwm_imx27_get_state() and provide the results?
>>
>> Note I only compile tested this change.
>
> Hi Uwe,
> I was just about to respond to the "pwm_bl on i.MX6Q broken on 5.4-RC1+"
> thread that I have a similar problem when you submitted this patch.
>
> So here are my few cents:
Once again with updated and more detailed debug messages.
> My setup is as follows:
> - imx6dl-yapp4-draco with i.MX6Solo
> - backlight is controlled with inverted PWM signal
> - max brightness level = 32, default brightness level set to 32 in DT.
>
> 1. Almost correct backlight behavior before 01ccf903edd6 ("pwm: Let
> pwm_get_state() return the last implemented state):
>
> - System boots to userspace and backlight is enabled all the time from
> power up.
>
- $ dmesg | grep pwm_
[ 1.761546] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0
[ 5.012352] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0
[ 5.021143] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 0
[ 5.030182] pwm_imx27_apply: period: 500000, duty: 500000, polarity: 1, enabled: 1
>
> - $ cat brightness
> 32
>
> - $ echo 32 > brightness # nothing happens, max. brightness
>
> - $ echo 1 > brightness # backlight goes down to lowest level
[ 93.976354] pwm_imx27_apply: period: 500000, duty: 7843, polarity: 1, enabled: 1
>
> - $ echo 0 > brightness # backlight goes up to max. level, this is
> # problem of the inverted PWM on i.MX we attempted
> # to solve some time ago.
[ 115.496350] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0
>
> 2. Backlight behavior on v5.4-rc3:
>
> - System boots to userspace and backlight is enabled all the time from
> power up.
>
- $ dmesg | grep pwm_
[ 1.774071] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0
[ 5.003961] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0
[ 5.012649] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0
[ 5.021694] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 0
[ 5.030732] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0
[ 5.039643] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 1
[ 5.049605] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 1
>
> - $ cat brightness
> 32
>
> - $ echo 32 > brightness # backlight goes down
[ 707.946970] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 1
[ 707.958551] pwm_imx27_get_state: period: 992970, duty: 992970, polarity: 0, enabled: 1
>
> - $ echo 1 > brightness # backlight goes up to high level
[ 757.516845] pwm_imx27_apply: period: 992970, duty: 15576, polarity: 0, enabled: 1
[ 757.528438] pwm_imx27_get_state: period: 992970, duty: 15576, polarity: 0, enabled: 1
>
> - $ echo 0 > brightness # backlight goes up to highest level
[ 783.386838] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 0
[ 783.398025] pwm_imx27_get_state: period: 496485, duty: 0, polarity: 0, enabled: 0
>
> So the behavior is clearly inverted to how it worked prior to 01ccf903edd6
> with the weird exception that the initial brightness level 32 is
> not applied.
>
> 3. Backlight behavior on v5.4-rc3 + this patch:
>
> - System boots with backlight enabled. In the middle of kernel boot
> backlight is disabled.
>
> - $ dmesg | grep state
[ 1.773099] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0
[ 5.002532] pwm_imx27_apply: period: 500000, duty: 0, polarity: 1, enabled: 0
[ 5.011263] pwm_imx27_get_state: period: 992970, duty: 0, polarity: 0, enabled: 0
[ 5.020307] pwm_imx27_apply: period: 992970, duty: 992970, polarity: 0, enabled: 1
[ 5.031066] pwm_imx27_get_state: period: 992970, duty: 992970, polarity: 0, enabled: 1
>
> - $ cat brightness
> 32
>
> - $ echo 32 > brightness # nothing happens, backlight is down
>
> - $ echo 1 > brightness # backlight goes to high level
[ 73.786926] pwm_imx27_apply: period: 992970, duty: 15576, polarity: 0, enabled: 1
[ 73.798469] pwm_imx27_get_state: period: 992970, duty: 15576, polarity: 0, enabled: 1
>
> - $ echo 0 > brightness # backlight goes to max brightness
[ 104.636908] pwm_imx27_apply: period: 992970, duty: 0, polarity: 0, enabled: 0
[ 104.648093] pwm_imx27_get_state: period: 496485, duty: 0, polarity: 0, enabled: 0
>
> Same behavior as (2) but the default state from DT is apparently applied.
>
> I only did this experiments. I did not delve into the code to track what is
> going on in there yet.
>
> Hopefully this helps you a bit and feel free to request other experiments,
> Michal
>
>> drivers/video/backlight/pwm_bl.c | 34 +++++++++++---------------------
>> 1 file changed, 12 insertions(+), 22 deletions(-)
>>
>> diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
>> index 746eebc411df..ddebd62b3978 100644
>> --- a/drivers/video/backlight/pwm_bl.c
>> +++ b/drivers/video/backlight/pwm_bl.c
>> @@ -42,10 +42,8 @@ struct pwm_bl_data {
>> static void pwm_backlight_power_on(struct pwm_bl_data *pb)
>> {
>> - struct pwm_state state;
>> int err;
>> - pwm_get_state(pb->pwm, &state);
>> if (pb->enabled)
>> return;
>> @@ -53,9 +51,6 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb)
>> if (err < 0)
>> dev_err(pb->dev, "failed to enable power supply\n");
>> - state.enabled = true;
>> - pwm_apply_state(pb->pwm, &state);
>> -
>> if (pb->post_pwm_on_delay)
>> msleep(pb->post_pwm_on_delay);
>> @@ -67,40 +62,27 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb)
>> static void pwm_backlight_power_off(struct pwm_bl_data *pb)
>> {
>> - struct pwm_state state;
>> -
>> - pwm_get_state(pb->pwm, &state);
>> - if (!pb->enabled)
>> - return;
>> -
>> if (pb->enable_gpio)
>> gpiod_set_value_cansleep(pb->enable_gpio, 0);
>> if (pb->pwm_off_delay)
>> msleep(pb->pwm_off_delay);
>> - state.enabled = false;
>> - state.duty_cycle = 0;
>> - pwm_apply_state(pb->pwm, &state);
>> -
>> regulator_disable(pb->power_supply);
>> pb->enabled = false;
>> }
>> -static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness)
>> +static int compute_duty_cycle(struct pwm_bl_data *pb, int brightness, struct pwm_state *state)
>> {
>> unsigned int lth = pb->lth_brightness;
>> - struct pwm_state state;
>> u64 duty_cycle;
>> - pwm_get_state(pb->pwm, &state);
>> -
>> if (pb->levels)
>> duty_cycle = pb->levels[brightness];
>> else
>> duty_cycle = brightness;
>> - duty_cycle *= state.period - lth;
>> + duty_cycle *= state->period - lth;
>> do_div(duty_cycle, pb->scale);
>> return duty_cycle + lth;
>> @@ -122,12 +104,20 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
>> if (brightness > 0) {
>> pwm_get_state(pb->pwm, &state);
>> - state.duty_cycle = compute_duty_cycle(pb, brightness);
>> + state.duty_cycle = compute_duty_cycle(pb, brightness, &state);
>> + state.enabled = true;
>> pwm_apply_state(pb->pwm, &state);
>> +
>> pwm_backlight_power_on(pb);
>> - } else
>> + } else {
>> pwm_backlight_power_off(pb);
>> + pwm_get_state(pb->pwm, &state);
>> + state.enabled = false;
>> + state.duty_cycle = 0;
>> + pwm_apply_state(pb->pwm, &state);
>> + }
>> +
>> if (pb->notify_after)
>> pb->notify_after(pb->dev, brightness);
>>
>
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